The present invention relates to a fuel cell system and a method of controlling the same, and more particularly, to a fuel cell system of a solid polymer electrolyte type and a method of controlling the same.
It has been proposed to use a fuel cell of a solid polymer electrolyte type that employs a polymer membrane as an electrolyte. The fuel cell of the solid polymer electrolyte type is advantageous over other fuel cells employing liquid electrolyte in that electrolyte is not leaked outside, and is highly expected as a power source in automotive vehicles. In the fuel cell of the solid polymer electrolyte type, the polymer membrane serving as electrolyte does not fully function as an ion conductor unless the polymer membrane is impregnated with moisture to some extent. To overcome this situation, it has been proposed to supply hydrogen gas with moisture to a fuel cell stack as disclosed in Japanese Patent Application Laid-Open Publication No. H7-263010.
In the event that moisture is supplied to the polymer membrane during operation of the fuel cell of the solid polymer electrolyte type, however, moisture is caused to remain in the polymer membrane by an excessive amount under conditions in that the fuel cell is operated at a low stoichiometric ratio. As a result, surplus moisture is left in the fuel cell, lowering electric power output produced by some fuel cells with a resultant difficulty caused in operating performance of the fuel cell.
In order to overcome such a situation, it is an effective practice to instantaneously exhaust surplus hydrogen gas from the fuel cell, which is not reacted therein, at an increased flow rate and to purge moisture with surplus hydrogen gas from the fuel cell.
In a purging process of moisture, it is thinkable to increase operating pressure of a fuel cell stack for instantaneously increasing the flow rate of hydrogen gas to be exhausted from the fuel cell stack. However, it is contemplated to increase an instantaneous flow rate of hydrogen gas to be exhausted from the fuel cell stack to remove moisture from the fuel cell stack by increasing the operating pressure of the fuel cell stack, resulting in an increase in electric power consumption that is undesirable for the fuel cell of the solid polymer electrolyte type that is intended to be used as the power source of the automotive vehicle.
Also, when a control system that controls the fuel cell of the solid polymer electrolyte type is modified to perform a purge treatment of moisture, the control system is caused to be complicated in structure and the whole structure of the control system is largely sized, with a resultant disadvantage caused in the fuel cell of the solid polymer electrolyte type that serves as the power source of the automotive vehicle.
The present invention has been made in view of the above-described investigation and has an object to provide a fuel cell system and a method controlling the same that are simple in structure and are enable to suitably operate a fuel cell of a solid polymer electrolyte type with a suitable purge treatment of moisture without causing remarkable increase in electric power consumption.
In the present invention, a fuel cell system is provided with: a fuel cell stack of a polymer electrolyte type; a hydrogen supply unit supplying hydrogen gas to the fuel cell stack; an air supply unit supplying air to the fuel cell stack; a hydrogen draw pump located at a downstream side of the fuel cell stack and drawing hydrogen gas from the fuel cell stack; a first hydrogen control valve located between the fuel cell stack and the hydrogen draw pump and having a controllable opening degree; and a control unit controlling operation of the fuel cell system such that, when the control unit discriminates that moisture in the fuel cell stack remains at an excessive level, the control unit increases drawing power of the hydrogen draw pump, decreases the opening degree of the first hydrogen control valve and subsequently opens the first hydrogen control valve while purging moisture from the fuel cell stack.
In other words, a fuel cell system of the present invention is provided with: a fuel cell stack of a polymer electrolyte type; a hydrogen supplying means for supplying hydrogen gas to the fuel cell stack; an air supplying means for supplying air to the fuel cell stack; a hydrogen drawing means for drawing hydrogen gas from the fuel cell stack; an adjusting means for adjusting an opening degree with respect to the hydrogen gas from the fuel cell stack; a controlling means for controlling operation of the fuel cell system such that, when the controlling means discriminates that moisture in the fuel cell stack remains at an excessive level, the controlling means increases drawing power of the hydrogen drawing means, decreases the opening degree of the adjusting means and subsequently opens the adjusting means while purging moisture from the fuel cell stack.
Besides, a method of controlling a fuel cell system of the present invention controls a fuel cell system having a fuel cell stack of a polymer electrolyte type, a hydrogen supply unit supplying hydrogen gas to the fuel cell stack, an air supply unit supplying air to the fuel cell stack, a hydrogen draw pump located at a downstream side of the fuel cell stack and drawing hydrogen gas from the fuel cell stack, a hydrogen control valve located between the fuel cell stack and the hydrogen draw pump and having a controllable opening degree. In the method, operation of the fuel cell system is controlled such that, when it is discriminated that moisture in the fuel cell stack remains at an excessive level, drawing power of the hydrogen draw pump is increased, the opening degree of the first hydrogen control valve is decreased and subsequently the first hydrogen control valve is opened while purging moisture from the fuel cell stack.
Other aspect and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.